Former Boston Mayor Thomas Menino has been diagnosed with cancer.

What Menino's Cancer Diagnosis Means

Former Boston Mayor Tom Menino, who days ago announced he has an advanced form of cancer that has spread to his liver and lymph nodes, finds himself among the small percentage of people whose cancer can't be tracked back to its origin.

It's rare but it happens, like in the case of Menino. Doctors find cancer somewhere in a patient’s body, but they can't figure out where the cancer originated.

"Cancers will often retain the features that they have or had wherever they started," said Dr. David Ryan, the clinical director of the Massachusetts General Hospital Cancer Center.

So prostate cancer, for instance, will look like prostate cancer, even if it is first found in, say, the liver.

"But every so often these cancers can all look the same under the microscope," Ryan said.

When that happens, doctors turn to a series of specialized tests, including molecular and DNA tests, to determine where the cancer originated. But when those tests are inconclusive we have what is called “cancer of unknown primary" — a diagnosis that Ryan said can be particularly hard for patients and their families.

"It’s very hard to get your arms or your mind wrapped around a diagnosis of cancer of unknown primary," Ryan said. "There’s no support groups, it’s very hard to find literature on the Internet or in books and so people feel a little bit lost and that makes it a little bit harder to take care of patients from that perspective."

But from a medical perspective, cancer of unknown primacy is no more mysterious to doctors than, say, breast cancer or lung cancer.

"Typically there are always physicians at every major cancer center that are the go-to people for cancer of unknown primary, and we have standard algorithms that we use and standard treatments that we have used over the years," Ryan said.

The treatment often includes chemotherapy, which the Menino says he started earlier this month at Dana Farber Cancer Institute. Ryan says everyone’s response to chemo is different and the prognosis can be difficult to assess.

"What makes it hard from a patient perspective, is it's hard to give statistics, because we don’t have the robust data sets that we have in breast cancer or colon cancer or prostate cancer," he said.

But Ryan stresses that despite a wealth of data about this particular form of the disease, one thing is certain: Cancer of unknown origin has — and can — be beaten.

"With cancer you never say never, and you never say always — and particularly for cancer of unknown primary, so the flip side of the unknowing is also the hope," he said.